1. Field of the Invention
This invention relates generally to semiconductor fabrication, and, more particularly, to dynamic adjustment of sensor and/or metrology sensitivities during semiconductor fabrication.
2. Description of the Related Art
Advanced process control (APC) systems are often used to coordinate operation of processing tools used to fabricate semiconductor devices. The processing tools may include photolithography steppers, etch tools, deposition tools, polishing tools, rapid thermal processing tools, ion implantation tools, and the like. Wafers (or wafer lots) are processed in the tools in a predetermined order and each processing tool modifies the wafers according to a particular operating recipe. For example, a photolithography stepper may be used to form a patterned layer of photoresist above the wafer. Features in the patterned layer of photoresist correspond to a plurality of features, e.g. gate electrode structures, which will ultimately be formed above the surface of the wafer.
The APC system typically includes devices for collecting data indicative of the physical and/or chemical state of the processing tool before, during, and/or after processing by the processing tools. The collected data indicative of the physical and/or chemical state of the processing tool is commonly referred to as process state information. For example, the process state information may include information indicative of abundances, concentrations, and/or mass-to-charge ratios associated with chemical species used in the processing tool. For another example, the process state information may include information indicative of one or more radiofrequency powers, spin speeds, pressures, and/or temperatures used in the processing tool. Process state information may be determined using data collected by a variety of well-known measurement devices including, but not limited to, optical emission spectroscopy devices, residual gas analyzers, mass spectrometers, and Fourier transform infrared devices.
The APC system may also include a variety of devices for collecting data indicative of the physical state of one or more wafers before, during, and/or after processing by the processing tools. The collected data indicative of the physical state of the wafer is commonly referred to as wafer state data. The collected wafer state data may be provided to the APC system, which may use the collected wafer state data to characterize the wafer and/or to detect faults associated with the processing. For example, a mean critical dimension associated with the various features may be indicative of a performance level of devices formed on the wafer and/or the wafer lot. If the wafer state data indicates that the mean critical dimension associated with the feature, e.g., a gate electrode feature, is on the lower end of an allowable range for such feature sizes, then this may indicate that the device formed on the wafer may exhibit relatively high performance levels. Higher performance devices may be sold at a higher price, thereby increasing the profitability of the manufacturing operation. However, the wafer state data may indicate that devices formed on the wafer and/or wafer lot have a relatively low performance level or are faulty if the mean critical dimension is near an upper end of the allowable range or falls outside of the allowable range.
Wafer state data may be collected by sensors incorporated within a processing tool, such as scatterometers, ellipsometers, and the like, in which case the wafer state data is referred to as in situ wafer state data. The in situ wafer state data may include measurements of a temperature of the wafer, a thickness of a layer of material formed above the wafer, a critical dimension of a feature formed above the wafer, or other characteristic parameters. Wafer sampling by the in situ sensors can increase the time spent by the wafer in the processing tool and so, in order to maintain a desired throughput, sensors usually perform gross metrology in which a small and/or isolated region on each wafer is sampled with relatively low sensitivity.
Wafer state data may also be collected by devices external to the processing tool, in which case the wafer state data is referred to as ex situ wafer state data. The ex situ wafer state data may include a thickness of a layer formed above the wafer, a critical dimension (CD) of a feature formed above the wafer, and the like. For example, an integrated metrology tool, i.e. a metrology tool that is coupled to a processing tool, may be used to collect ex situ wafer state data from a subset of the wafers that have been processed in the processing tool. Relative to sensors included within the processing tool, integrated metrology tools typically operate at a higher sensitivity and so perform higher accuracy measurements and/or measurements at a higher granularity. However, at least in part to maintain a desired throughput, the integrated metrology tools perform these measurements on a smaller number of wafers and/or on a smaller area on the wafer. For another example, a stand-alone metrology tool, i.e. a metrology tool that is physically separate from the processing tools, may be used to collect ex situ wafer state data from a subset of the wafers that have been processed in the processing tool. Compared to integrated metrology tools, stand-alone metrology tools typically operate at a higher sensitivity and so perform higher accuracy and/or higher granularity measurements, but on a smaller number of wafers and/or on a smaller area of the wafer.
Sensitivities of metrology tools are typically determined prior to beginning a fabrication run and may only be changed by interrupting the run and manually changing the sensitivity. Using predetermined metrology sensitivities may limit the ability of the advanced process control system to control the quality of the fabricated semiconductor devices. The predetermined metrology sensitivities may also limit the ability of the advanced process control system to track, analyze, and/or diagnose causes of killer defects. Moreover, changing the sensitivities manually may reduce the efficiency and/or throughput of the advanced process control system.
The present invention is directed to addressing the effects of one or more of the problems set forth above.